1. Field of the Invention
The present invention pertains to a flat-tubular solid oxide fuel cell stack using an anode as a supported body and a method of fabricating the stack. More particularly, the present invention relates to an anode-supported flat-tubular solid oxide fuel cell stack, which includes an anode-supported tube having semi-cylinder parts and plate parts, thereby having a combined structure of a tube-type and a plate-type anode-supported body, and a method of fabricating the stack. The anode-supported flat-tubular solid oxide fuel cell stack is advantageous in that fuel cells constituting the stack are easily sealed, and have excellent resistance to heat stress and improved power density per unit area.
2. Description of the Related Art
A fuel cell is a highly efficiency clean generator, in which hydrogen contained in a hydrocarbon based material such as natural gas, coal gas, or methanol, is electrochemically reacted with oxygen contained in air to produce electric energy. It is classified into alkaline, phosphoric acid, molten carbonate, solid oxide, and polymer fuel cells.
Generally, the phosphoric acid fuel cell using a phosphoric acid electrolyte is referred to as a first generation fuel cell, in which hydrogen gas mostly containing hydrogen reformed from fossil fuel and oxygen contained in air are used as fuel, the high temperature molten carbonate fuel cell using molten carbonate as the electrolyte and operating at about 650° C. is referred to as a second generation fuel cell, and the solid oxide fuel cell (SOFC) operating at a relatively higher temperature and generating the most efficient electricity is referred to as a third generation fuel cell.
The third generation fuel cell, the solid oxide fuel cell, was developed after the phosphoric acid fuel cell (PAFC) and the molten carbonate fuel cell (MCFC), but it is expected that the solid oxide fuel cell will be rapidly commercialized in subsequent to PAFC and MCFC due to a rapid development of material technology. Additionally, the solid oxide fuel cell is operated at a high temperature ranging from 600 to 1000° C., and has advantages in that it is the most efficient of existing fuel cells, there are few pollutants discharged, a fuel reformer is not necessary, and a combined power generation is feasible.
The solid oxide fuel cell is generally classified into a tube type fuel cell, a plate type fuel cell, and a single body type fuel cell according to a shape of the solid oxide fuel cell. Of them, currently, the tube and plate type fuel cell are mostly studied, a technology of the tube type fuel cell is considered as the most advanced technology, and a study of the plate type fuel cell is more advanced than that of the single body type fuel cell. As for the tube type fuel cell, an air electrode support type fuel cell has been developed in the USA and Japan, and a self-supporting film type fuel cell comprising an electrolyte as a support and an anode-supported plate fuel cell has been developed as the plate type fuel cell.
The plate type solid oxide fuel cell has a higher current density than a disk type fuel cell, but has disadvantages in that a large-sized plate type fuel cell necessarily needed to produce a large capacity fuel cell is difficult to produce using the plate type solid oxide fuel cell due to several problems, such as sealing gas and thermal shock due to a difference of thermal equilibrium coefficient between constituents of the fuel cell.
In comparison with the plate type solid oxide fuel cell, the tube type solid oxide fuel cell is advantageous in that unit cells constituting the stack are easily sealed, resistance to heat stress and mechanical strength of the stack are high, thereby the tube type solid oxide fuel cell is considered to be an excellent technology by which the large-sized fuel cell can be most easily produced. However, the tube type solid oxide fuel cell has disadvantages in that the tube type solid oxide fuel cell has a lower power density per unit area than the plate type solid oxide fuel cell, and the production costs of the tube type solid oxide fuel cell are relatively high.
Meanwhile, a conventional tube type fuel cell is an air electrode-supported fuel cell using an air electrode as the support of the fuel cell, and the production costs of the fuel cell are increased because raw materials for the air electrode such as La and Mn, are very expensive, and production of LSM (LaSrMnO3) is difficult. In addition, the unit cell is low in mechanical strength and does not withstand impact because the air electrode acting as the support is made of ceramics while an anode is made of cermet consisting of metals and ceramics.
Furthermore, an electrolyte layer is coated on a surface of the air electrode-supported tube according to a process requiring high coating costs during fabrication of the conventional air electrode-supported tubular solid oxide fuel cell, thus the conventional air electrode-supported tubular solid oxide fuel cell is disadvantageous in terms of economic efficiency.
In other words, the air electrode is fragile because the air electrode is made of high-priced ceramics, such as La, which is used as the support in the conventional air electrode-supported tubular solid oxide fuel cell, the strength of the air electrode is reduced due to a chemical reaction, in the ceramic structure constituting the air electrode, at high temperatures, and the fuel cell price is increased because the electrolyte layer is formed on the surface of the air electrode by use of the very costly EVD process.
Furthermore, the electrolyte and the anode formed on a surface of the sintered air electrode-supported tube are co-sintered at high temperatures, and thus activity of the air electrode is reduced and an efficiency of the fuel cell is lowered.
To avoid the above disadvantages of the air electrode-supported solid oxide fuel cell, the anode-supported tubular solid oxide fuel cell using the anode as the support has been developed. The anode-supported tube used in the anode-supported tubular solid oxide fuel cell satisfies characteristics required by the electrode as well as acts as the support, and is advantageous in that co-sintering is feasible because reactivity is low between the support and the electrolyte layer, and a stable fuel cell stack can be fabricated due to high mechanical strength of the anode.
In addition, the anode-supported tube has sufficient pores therein, a fuel provision is not limited because a continuous pore distribution is formed in the anode-supported tube, and current flow is smooth due to high electric conductivity and production costs of the fuel cell are low.
When the anode-supported tubular solid oxide fuel cell is produced, the most important factor affecting productivity is a process of coating the electrolyte layer on the surface of the anode-supported tube.
In detail, a conductivity of YSZ, which is most widely used as the electrolyte in the solid oxide fuel cell, is about 10−1 S/cm at 1000° C., and the electrolyte layer should have a thickness of about 30 μm or lower and be very dense because the lower an operating temperature of the fuel cell is, the lower the conductivity is or the higher a resistance is.
The anode-supported tubular solid oxide fuel cell has a disadvantage in that the very thin and dense electrolyte layer should be formed on the surface of the porous anode-supported tube with a wide surface area. Therefore, recently, many studies have been initiated in order to develop a process of efficiently forming an excellent electrolyte layer.
When the electrolyte layer is formed by a physical and chemical vapor deposition process using a vacuum such as an EVD process or a plasma spray coating process, a dense and thin electrolyte layer can be formed, but equipment used to form the electrolyte layer is undesirably large and the reaction time is excessively long. In addition, the above processes are not suitable to produce the fuel cell in commercial quantities because a few of the unit cell are deposited by the electrolyte at one time.